[gtk: 2/3] gdkframeclockidle: Don't permanently skew frame time

[Matthias Clasen <matthiasc src gnome org>]

commit 0bf4c2420dbc318052a9e561af6bc2eefdb9e7a1
Author: Chris Williams <chrisaw1990 gmail com>
Date:   Tue Apr 9 16:26:25 2019 -0400

    gdkframeclockidle: Don't permanently skew frame time
    
    Since commit 3b2f9395, the frame time may be set into the future, so
    only ensure monotonicity, and don't store the offset. This prevents the
    frame time from becoming out of sync with g_get_monotonic_time().
    
    Fixes #1612

 gdk/gdkframeclockidle.c | 20 ++++----------------
 1 file changed, 4 insertions(+), 16 deletions(-)
---
diff --git a/gdk/gdkframeclockidle.c b/gdk/gdkframeclockidle.c
index 4a6ea10817..0e9bc3b1b7 100644
--- a/gdk/gdkframeclockidle.c
+++ b/gdk/gdkframeclockidle.c
@@ -38,8 +38,6 @@
 
 struct _GdkFrameClockIdlePrivate
 {
-  /* timer_base is used to avoid ever going backward */
-  gint64 timer_base;
   gint64 frame_time;
   gint64 min_next_frame_time;
   gint64 sleep_serial;
@@ -160,22 +158,12 @@ compute_frame_time (GdkFrameClockIdle *idle)
 {
   GdkFrameClockIdlePrivate *priv = idle->priv;
   gint64 computed_frame_time;
-  gint64 elapsed;
 
-  elapsed = g_get_monotonic_time () + priv->timer_base;
-  if (elapsed < priv->frame_time)
-    {
-      /* clock went backward. adapt to that by forevermore increasing
-       * timer_base.  For now, assume we've gone forward in time 1ms.
-       */
-      /* hmm. just fix GTimer? */
+  computed_frame_time = g_get_monotonic_time ();
+
+  /* ensure monotonicity of frame time */
+  if (computed_frame_time <= priv->frame_time)
       computed_frame_time = priv->frame_time + 1;
-      priv->timer_base += (priv->frame_time - elapsed) + 1;
-    }
-  else
-    {
-      computed_frame_time = elapsed;
-    }
 
   return computed_frame_time;
 }